The lobby area of the DT OldTown is more expansive and the breakfast there is definitely better than the Sirkeci hotel. The tram stop is also more convenient, but that is relative - the tram is pretty convenient from the DT Sirkeci as well.

The rooms are similar - no / minimal views. However, I got upgraded to a nice suite - so my experience at the Sirkeci was superior.

Since the Sirkeci is usually much cheaper than the Old Town hotel, I would have no desitation in opting for it on a future trip. Something else - since the DT Sirkeci is less conspicuous than the Old Town, I feel it would be a less attractiv target for the really bad guys.